Electric discharge projection lamp



April 23, 1968 J. K. TAILLON 3,379,868

ELECTRIC DISCHARGE PROJECTION LAMP Filed Dec. 10, 1965 Fig 1. 4

lnven tor: John K. TaiLLon b5 HIS Adrto neg United States Patent 01 ice 3,379,868 Patented Apr. 23, 1968 3,379,868 ELECTRIC DISCHARGE PROEECTKQN LAMP John K. Taillon, Cleveland, Ohio, assignor to General Electric Company, a corporation of New York Filed Dec. 10, 1965, Ser. No. 513,026 8 Claims. (Cl. 240-11.:1)

ABSTRACT OF THE DISCLOSURE A projection lamp unit comprising a light concentrating reflector having a locating flange at its rim and wherein an arc tube is transversely mounted with the interelectrode gap located at one focus. The are tube inleads project through apertures in the reflector wall on a line transverse to the optical axis. One end of the arc tube is fixed to the reflector, preferably being set in cement. At the other end, a slidable sleeve is provided around the inlead and is cemented to the reflector.

This invention relates to an electric discharge projection lamp particularly suitable for slide and movie projectors.

The photographic projection system most widely used up to the present comprises an incandescent filament lamp, a spherical reflector mounted behind the lamp to reflect rearwardly directed light back upon the filament, and a condensing lens in front of the lamp to concentrate the light upon the aperture in the film gate. Light transmitted through the film at the aperture then passes through a projection lens which focuses it on a screen for enlargement of the film image. In projection systems making use of a discharge lamp as the light source, the same kind of optical system has generally been used involving a reflector and condensing lens.

In copending application Ser. No. 279,270, filed May 29, 1963, of Elmer G. Fridrich entitled, Electric Discharge Lamp Manufacture, now Patent 3,305,289, there is disclosed a compact source short are gap electric discharge lamp comprising a quartz envelope having a relatively thick-walled central bulb portion and solid neck portions on each side through which extend hermetically sealed foil inleads having electrodes attached thereto projecting into the bulb portion. The ionizable medium within the lamp includes a halogen or a metal halide such as indium or gallium iodide. The electrodes consist of slender tungsten rods and the lamp operates with the electrode tips molten or close to the melting point of tungsten. The discharge volume of the bulb has been aptly described as being about the size of a drop of water. By way of example, an indium iodide lamp of this kind rated at 300 watts input is about 5 centimeters in overall length with the diameter of the bulb being about 9 millimeters and the transverse diameter of the discharge space being about 2.5 millimeters.

The compactness and the spectral quality of the indium iodide arc lamp make it very suitable for the light source in a projection system. However the lamp operates extremely hot and it is quite sensitive to temperature. Excessively high temperatures cause seal failures While low temperatures cause degradation of spectral output. Mounting the arc tube and accurately locating it with respect to the optical system are also problems, and finally the arc tube has been known to have violent seal failures.

The object of the invention is to provide a projection lamp incorporating a compact source are tube of the kind described mounted in a reflector through which the light source may be accurately placed relative to a film aperture in a projection system. A unit is desired which achieves a high degree of light utilization, permits good control of arc tube seal temperature, and which is at the same time compact and rugged.

In accordance with a preferred embodiment of the invention, these objectives are achieved in a novel projection lamp unit comprising a light concentrating reflector, preferably of glass and suitably of generally ellipsoidal shape, having a locating flange at its mouth or rim and wherein the arc tube is transversely mounted with the interelectrode gap located substantially at one focus. The arc tube inleads project through apertures in the reflector Wall on a line transverse to the optical axis and passing through the focus. One arc tube end is rigidly fastened or fixed to the reflector, preferably being set in cement. The other end of the arc tube is not cemented in place but instead a slidable sleeve is provided around the inlead and only the sleeve is cemented and fixed to the reflector. This permits the quartz arc tube to expand at a diiferent rate from the glass reflector without subjecting either to excessive strain. Where a metal reflector is used, the greatly different rates of expansion make the slidable joint all the more necessary.

Further features and advantages of the invention will appear from the following detailed description to be read with the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 is a front end view of a projection lamp embodying the invention.

FIG. 2 is a side view of the same lamp.

FIG. 3 is a sectional plan view of the lamp of FIG. I seen along line 3-3 in FIG. 1.

Referring to the drawing, there is illustrated a projection lamp unit 1 comprising a reflector portion 2, preferably made of glass and molded to an ellipsoidal shape, having a light source in the form of an arc tube 3 at its near focus f1. The ellipsoidal shape of the reflector concentrates the light at the far focus which conveniently may be located in front of the rim seating plane of the light unit at a distance about equal to the diameter across the rim. The reflector is provided with two flange segments 4, 5 at its rim by which the lamp is supported against a cooperating seating shoulder 6 in a support member 7 as shown in FIG. 2. By using a glass base reflector, dimensional stability is assured. Also it may be coated on its inner surface with a reflecting coating 8 consisting of a known type of multiple layer interference film which is highly reflective of visible light but transmissive of heat or infrared radiation. A metal reflector with a conventional mirror finish may also be used.

The light source or are tube 3 comprises a quartz envelope having a generally spherical central portion or bulb 10 provided with generally cylindrical extensions or necks 11, 12. The electrodes 13, 14 consist of short lengths of tungsten wire which are welded to molybdenum foils 15, 16, the foils in turn being welded to inleads 17, 18. The foils are wetted by the quartz of the necks to provide the hermetic seals. The illustrated lamp is intended for direct current operation and the anode 13 is of tungsten wire larger in diameter than the cathode 14. In a lamp for A-C operation, the two electrodes would be of the same size and the necks would be equal in length. The lamp operates with the electrodes close to the melting point of tungsten and may operate with substantially molten tips resulting in the rounding and balling of the electrode ends during operation. The lamp contains an ionizable filling which includes an inert gas such as argon and a halogen or metal halide such as indium iodide. By way of example, the over-all length of the arc tube quartz body may be 5 centimeters and the diameter of the bulb portion may be 9 millimeters with the internal diameter of the discharge 3 space being about 25 millimeters. The bulb has no exhaust tip and is optically perfect; the tube is symmetrical end for end except for the anode neck 11 being longer than the cathode neck 12.

Lune segments are cut-away from the sides of the reflector 2 leaving flat vertical side walls 19, 20 which are chord-like in end view as seen in FIG. 1 and tapered rearward slightly towards the axis in plan section as seen in FIG. 3. In the front half of the reflector, the chord-like side walls 19, 20 cut into the normal elliptical curve of the reflector surface but in the rear half, they are extended into shouders 21, 22 defining generally wedge-like expansions within the reflector. Within the shoulder expansions, the glass is built up into collars 23, 24 about lateral apertures 25, 26 through the chord walls; the apertures are centered on a line passing through focus f1 and transverse to th optical axis.

Arc tube 3 is mounted on the transverse axis with neck 11 projecting fully into aperture 25 and set in a glassy cement 27 which fills the volume of the aperture and of a ceramic cap 28 which is placed over the opening. An insulated wire lead 29 is welded to the end of inlead l7 and emerges from cap 28 through a small side aperture at the rear. Neck 12 at the cathode end of the arc tube does not penetrate into aperture 26; a snug-fitting sleeve 31 is provided around inlead 13 and the sleeve projects into lateral aperture 26 and is there set in cement 27. An insulated wire lead 32 is connected to the end of sleeve 31 and emerges from cap 33 through a side aperture to the rear. Before cement 27 sets hard, arc tube 3 is adjusted in all directions to optically center the are at the near focus f1 while the lamp unit as a whole is accurately located relative to the projection system by the flange segments 4, 5. Preferably a cement is used which sets quickly under heat and bonds to both the glass reflector and the quartz arc tube. One suitable cement comprises primarily fine alumina and calcined kaolin along with minor additions of disodium phosphate and trialuminum phosphate mixed with phosphoric acid to form a paste.

By setting one end of arc tube 3 in cement so that it is rigidly fixed to reflector 2, a projection lamp unit results wherein the arc tube is accurately located in an optical reference system. When subsequently the unit is inserted into a socket properly accommodating flange segments 4, 5, it will provide the desired light at the film gate without further adjustment. At the other end of the arc tube, the inlead is slidably engaged in sleeve 31. This permits differential expansion of the quartz arc tube having a. low coefiicient of expansion and of the glass reflector having a relatively high coefiiciently of expansion, without subjecting the parts to excessive strain. At the same time the displacement of the interelectrode gap relative to the re flector focus as a result of differential thermal expansion is too slight to be of any consequence in the optics of the system.

The light output of the arc tube is not spherically uniform but falls off along the axis of the necks, that is along the mounting axis of the arc tube which is transverse to the optical axis of the reflector. This fact is taken advantage of by the lamp unit of the invention wherein the shoulder expansions are placed in the regions where the light output from the arc tube is a minimum. Therefore the departure of the reflector from its normal ellipsoidal configuration in the regions of chord walls 19, 2t) and of shoulder expansions 21, 22 has minimum effect upon the optical performance of the system. The end result is an extremely compact lamp unit which is very eflicient in concentrating light at the film gate.

The depth of the reflector and the location of the arc tube well within it assure a high degree of utilization of generated light required to be concentrated at the film gate aperture. At the same time the reflector provides a thermal barrier protecting the rest of the equipment from the high lamp temperatures. Cooling air is prevented from impinging directly on the arc tube body and cooling it down to a temperature where the lamp would operate unsatisfactorily. Finally, in the event of a defective lamp and a violent seal failure, the reflector serves as a shield protecting the projector or other equipment with which the unit is associated.

The embodiment of the invention which has been described and illustrated is intended by way of example only, and the scope of the invention is to be determined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge projection lamp unit comprising a concave light concentrating reflector having a locating flange at its rim and a pair of opposed lateral apertures in its Wall on a line normal to the optical axis and passing substantially through its focus, and a compact source are tube transversely mounted in said reflector on said normal line, one end of said are tube being rigidly and permanently fastened to said reflector within one aperture, the other end of said are tube having an inlead fitting in a sliding bushing fastened to said reflector Within the other aperture.

2. A lamp unit as defined in claim 1 wherein the reflector is of vitreous material.

3. A lamp unit as defined in claim 1 wherein the rigidly fastened end of said are tube is set in cement located in said one aperture and adherent to said reflector.

4. A lamp unit as defined in claim 1 wherein the reflector is made of vitreous material and has locating flange segments at its rim and opposed flat chord-like walls departing from the normal curve of the reflector and extended into shoulders defining wedge-like expansions, the lateral apertures being provided in said expansions.

5. A lamp unit as defined in claim 4 wherein the arc tube is made of quartz and has a central bulb portion and linear necks extending therefrom, one neck extending into said one aperture and being set in cement therein.

6. A lamp unit as defined in claim 4 wherein the arc tube is made of quartz and has a central bulb portion and linear necks extending therefrom, one neck extending into said one aperture and being set in cement therein, and the other neck having an inlead fitting in a sliding bushing set in cement in said other aperture.

7. A lamp unit as defined in claim 6 including protective end caps mounted over said apertures and set in the cement filling the apertures.

8. A lamp unit as defined in claim 7 including insulated wire leads connected to said are tube within said apertures and emerging through apertures in said end caps.

References Cited UNITED STATES PATENTS 2,467,687 4/1949 Noel 2401l.4 2,791,679 5/1957 Hierholzev ct a1. 240-114 3,099,403 7/1963 Strawick 240--47 3,152,764 10/1964 Rice 24011.2 X 3,237,005 2/1966 Norton 240-112 FOREIGN PATENTS 1,199,206 8/1965 Germany.

NORTON ANSHER, Primary Examiner.

W. M. FRYE, R. M. SHEER, Asrixmn! Examine/s. 

